Time-of-flight MR flow imaging: selective saturation recovery with gradient refocusing

A novel magnetic resonance flow-imaging technique is presented and its suitability evaluated for both qualitative and quantitative imaging of flow. The method is derived from a selective saturation-recovery scheme consisting of a tagging and detection pulse followed by a bipolar read gradient. The detrimental phase effects causing signal loss at fast flow are shown to be greatly reduced because of the absence of a 180 degrees pulse and its associated section-selection gradient. The second loss mechanism intrinsic to 180 degrees spin echoes, the washout of excited spins between excitation and detection pulse, likewise is not present with the discussed technique. Assuming a parabolic flow profile, the authors calculated the signal evolution curve and found it to be in agreement with the experimental washout curve. The technique is shown to provide high-intensity signals for arteries such as carotid and vertebral arteries. Arteries and veins can be differentiated by judiciously choosing interpulse intervals or by alternating selective and nonselective tagging pulses.